Organic electroluminescence means converts electrical energy into light energy by using organic materials. Its principle is explained as follows. When voltage is applied between an anode and a cathode while aligning an organic material layer between the anode and the cathode, holes are injected into the organic material layer from the anode, and electrons are injected into the organic material layer from the cathode. When the injected holes couple with the injected electrons, excitons may be formed and such excitons fall to the ground state to generate light. Such generated light is emitted through an anode, a cathode or both electrodes. Generally, an organic light-emitting device may be classified as a top emission type, bottom emission type and bilateral emission type according to the emitting direction of light.
Recently, research has been actively carried out for preparing displays or illumination units by using such electroluminescent phenomenon. In addition, techniques for depositing organic material layers in the form of a single layer to multi-layers are being actively studied in order to achieve effective organic light-emitting devices. Most currently available organic light-emitting devices include electrode layers and organic material layers deposited in the form of a planar structure. An organic light-emitting device having a planar multi-layer structure comprising electrode layers, and organic material multi-layers including a hole injection layer (103), a hole transport layer (104), a light-emitting layer (105), and an electron transport layer (106) as shown in FIG. 1, has been widely used.
The light generated from a light-emitting layer of the organic light-emitting device in FIG. 1 may pass through two different paths. Namely, the light may be emitted out of the organic light-emitting device through a transparent anode layer and glass substrate or may remain in the organic light-emitting device by being reflected entirely from the glass substrate surface or the anode surface. At this time, the amount of the light emitted out of the organic light-emitting device is only about ½n2 among the light generated from the light-emitting layer (wherein, n is the refractive index of an organic material layer). If the refractive index of the organic material layer is 1.7, less than 17% of the light generated from the device can be emitted out of the organic light-emitting device.
To solve the above problem and emit a large amount of light out of the organic light-emitting device, an organic light-emitting device including a non-planar layer, i.e., non-planar structure, has been suggested. The organic light-emitting device having a non-planar structure can be prepared through the following two methods.
According to a first method, a photonic crystal having a gravure pattern is formed on a glass substrate through a photolithography process before a transparent anode is deposited on the glass substrate (see U.S. Pat. No. 6,630,684 and Appl. Phys. Lett. 82, 3779 issued in 2003 by Y. Lee et al.), or a corrugated pattern is formed on the glass substrate by using an interference of light (see WO 2000/70691 and Adv. Mater. 13, 123 issued in 2001 by B. J. Matterson et al.), for improving light-emitting efficiency. In detail, the former deposits an anode layer on the glass substrate, after forming the photonic crystal on the glass substrate and flattening the surface thereon by using SiNx. The latter deposits an electrode layer and an organic material layer on the glass substrate while maintaining a corrugated pattern, after forming the corrugated pattern of transparent polymer on the glass substrate by using photoresist materials and an interference of light.
According to a second method, after preparing an organic light-emitting device having a planar structure as shown in FIG. 1, a micro-sized lens structure (see WO 2003/007663 and J. Appl. Phys. 91, 3324 issued in 2002 by S. Moller et al.) or a millimeter-sized lens structure (see WO 2001/33598) is attached to a surface of a glass substrate of the organic light-emitting device, thereby improving the light-emitting efficiency of the device.
The above two methods can improve the light-emitting efficiency of the light-emitting device. However, the above two methods cause problems when they are applied to an available product.
The first method uses the photolithography process, so it may be impossible to economically form the photonic crystal structure or the corrugated structure over a large-sized area. That is, in order to prepare the light-emitting device using the photonic crystal structure, it is necessary to sequentially carry out a deposition process, a photolithography process, and an etching process. At this time, the substrate must be processed at least two times under a vacuum state. In addition, in order to prepare the light-emitting device using the corrugated structure, it is necessary to perform the photolithography process by using an interference of light. However, the photolithography process is not adaptable for forming a uniform corrugated structure over a substrate having an area more than a few cm2.
The second method has limitations when it is applied to a display because the lens structure has a size in the range of about a few micrometers to a few millimeters. In addition, the second method is not adaptable for a large-sized area due to preparation work thereof. According to the lens structure disclosed in WO 2003/007663, a minimum surface size of the lens structure is defined as a few μm such that the minimum surface size of the lens structure must be larger than the maximum wavelength of visible ray emitted from the organic light-emitting device. In addition, according to the lens structure disclosed in WO 2001/33598, the size of the lens structure must be larger than the size of one unit of an organic light-emitting device.